The present invention relates to an image reader capable of obtaining image data for an electrophotographic image forming apparatus, for example, and the image forming apparatus provided with the image reader.
A image reader (scanner) for obtaining image data for an image forming apparatus comprises a transparent document holder (document glass), illumination unit, CCD sensor, optical set, etc. The document glass is a substantially rectangular plate that serves to hold an object of reading, such as a sheetlike document, book, or cubic body. The illumination unit, which includes an illuminating lamp and a reflector, illuminates the object of reading set on the document glass. The CCD sensor receives reflected light from the object of reading that is illuminated with illumination light from the illumination unit, and converts an image contained in the object of reading into an electrical signal. The optical set includes a plurality of mirrors and reduced-scale lenses that are interposed between the document glass and the CCD sensor. An analog electrophotographic apparatus is designed so that an exposure mirror and a photoconductor drum are provided in place of the CCD sensor and that reflected light from the object of reading is guided directly to the drum. In this apparatus, a document glass (document holder), illumination unit, and optical set are arranged in like manner.
In the scanner (image reader) of this type, the illumination unit and a first mirror, which is located near the illumination unit and serves to reflect the reflected light from the object of reading in a given direction, are assembled together as a first carriage, which is movable along the document glass. Further, second and third mirrors, which serve successively to return the reflected light from the first mirror toward the CCD sensor, are arranged between the first mirror and the CCD sensor. They are assembled together as a second carriage, which is movable without changing its position relative to the first carriage.
The first and second carriages are moved so that the speed of the second carriage is half that of the first carriage, by means of a motor for generating driving force, a wire or belt for transmitting the driving force of the motor to the first and second carriages, and a reduction gear mechanism that reduces the rotational frequency of the motor as it adjusts the torque to a given value.
The reduction gear mechanism is composed of, for example, a motor pulley mounted on the shaft of the motor, a plurality of reduction pulleys arranged between the wire (or belt) and the motor pulley and capable of further reducing the driving force (rotational frequency) supplied to the motor pulley, and a plurality of timing belts or gear trains for transmitting the driving force between the reduction pulleys.
Thus, the image reader is a continuous physical system that includes a plurality of movable parts and movable components, such as the motor, wire or belt, first and second carriages, reduction gear mechanisms, etc. These parts and components have their respective natural frequencies that depend on their materials, shapes, methods of assembly, etc. It is known that they resonate to one another if their natural frequencies are equal.
If any oscillation is caused during the image reading operation of the image reader, then the quality of read image data will inevitably be lowered. For example, the image data quality may be lowered by a longitudinal-line fluctuation such that at least one of the carriages are staggered or moved in zigzags so that the CCD sensor inevitably reads image data as an image that oscillates in the advancing direction of the carriage.
At the time of shipment, the movable parts and components are assembled, adjusted, or shaped in an optimum state without resonation of the natural frequencies. Due to aging or prolonged use, however, the respective natural frequencies of the movable parts and components may change and undergo resonation.
If the quality of the read image data is lowered by resonance, it may possibly be improved by adjusting the tension on the wire or the torque of the drive motor. In most cases, however, the adjustment takes a lot of time. In many cases, moreover, the adjustment cannot improve the data quality. In these cases, it is necessary to recover and readjust the apparatus in the production line or plant or replace its defective parts or components.